This invention relates to dielectric substrates and, in particular, to thin dielectric substrates for use in circuit laminates and circuits, including multilayer circuits.
As used herein, a circuit material is an article used in the manufacture of circuits and multilayer circuits, and includes circuit laminates, bond plies, dielectric materials, conductive layers, resin coated conductive layers, and cover films.
Circuit laminates can have a conductive layer fixedly attached to a dielectric layer. When a second conductive layer is disposed on the other side of the dielectric layer, the material is often referred to as a double clad circuit laminate. Patterning a conductive layer of a circuit laminate, for example by etching, provides a circuit, or in the case of a double clad circuit laminate, a double clad circuit. One or both of the conductive layers of a double clad laminate can be processed to provide circuit layers.
The aforementioned circuit materials and circuits can be combined in various configurations to provide multilayer circuits. “Multilayer circuits” as used herein refers to materials having at least two dielectric layers and at least two conductive layers, wherein at least one of the conductive layers is circuitized, and is inclusive of both subassemblies used to form finished circuits and the finished circuits themselves.
In one simple form, a multilayer circuit includes a double clad circuit and a resin coated conductive layer, wherein the dielectric material of the resin coated conductive layer is disposed on a circuit layer of the double clad circuit. In another simple form, a multilayer circuit includes a first circuit and a second circuit joined by a bond ply, to provide adhesion, disposed between the circuit layer of the first circuit and the dielectric substrate of the second circuit. Typically, such multilayer circuits are formed by laminating the circuit(s) and/or circuit material(s) in proper alignment using heat and/or pressure. In place of a conductive layer bonded to a circuit with a bond ply, the multilayer circuit can include a resin coated conductive layer bonded directly to the outer layer of a circuit. In such multilayer structures, after lamination, known hole forming and plating technologies can be used to produce useful electrical pathways between conductive layers.
A dielectric layer (also referred to as a “substrate” for attachment to another layer, for example a conductive layer, or a “circuit substrate” for supporting a circuit) can comprise a fibrous mat or other form of reinforcement. Suitable fibrous mat reinforcement can be composed of glass fibers or, alternatively, of polymeric fibers having good dielectric properties, such as aromatic polyamides (“aramids”).
A liquid crystalline polymer in the form of a non-woven fibrous mat is commercially available from Kuraray America Inc., New York, N.Y., under the trade name VECRUS. Liquid crystalline polymer mats are described in U.S. Pat. No. 6,229,096, which is incorporated herein by reference in its entirety. The mats described therein comprise liquid crystalline fibers and a binder, wherein the fiber-binder combination is provided with sufficient porosity to allow infiltration of a resin composition. For example, U.S. Pat. No. 7,524,388 discloses a method of forming a reinforced dielectric substrate comprising contacting a liquid crystalline polymer fibrous mat, having a thickness of 5 mils (127 μm) or less, with a resin composition to form a dielectric composite layer, wherein the contacting is carried out under vacuum, followed by pressure. The method is referred to as vacuum pressure impregnation (VPI).
While there are a variety of reinforced substrate materials available today, for example FR4 epoxy glass laminates and the like, there is a growing demand for improved and thinner reinforced substrate materials for high performance (high frequency) applications, that is, applications operating at 1 gigahertz (GHz) or higher. As the complexity of multilayer circuits increases, there is an incentive to reduce the thickness of the dielectric layers of multilayer circuits. Thinner dielectric layers enable the addition of more layers of circuitry, enable the weight and dimensions of the circuit boards to be as low as possible, and allow addition of more interconnect circuitry to be incorporated into a single board. Thin ultra-low-loss materials are needed for digital applications in particular.
It is difficult, however, to achieve thinner dielectric layers and still maintain good mechanical and electrical properties. The thickness of a laminate comprising a glass fabric is determined by the glass fabric. As the thickness of the glass fabric goes down, the strength of the fabric is reduced, which can lead to distortion of the fabric during the impregnation process in which the reinforced substrate is produced. The properties of the fabric can also be inconsistent at the “knuckles,” where fibers overlap or cross over, since there would be more glass at the knuckles than at other places. As the laminate becomes increasingly thinner, the dielectric properties at these knuckles can become increasingly worse than at other parts of the laminate.
Non-woven fibrous mats have been considered as one possible solution for this problem. Such non-woven fibrous mats comprising chopped fibers, however, tend to have insufficient or low strength. Furthermore, they can also exhibit a mismatched CTE (coefficient of thermal expansion) along the x- and y-axes. Thus, while non-woven fibers can have better electrical properties than glass fibers, the non-woven fibrous mats can be very anisotropic. In fact, the CTEs can be quite different in x- and y-directions, which can lead to warping of the substrates during lamination.
Furthermore, very thin fibrous mats, less than or equal to about 4 mils (100 μm), can be prone to dimensional distortion when placed under mechanical stresses, for example, the stresses associated with the manufacture of reinforced prepregs and circuit laminates. The process of impregnating the mat, curing, heating, pressing, rolling, laminating, cutting, and the like can result in dimensional distortion in the x-y plane, thinning (which results in non-uniform thickness), and even tearing. Non-woven fibrous mats comprised of chopped fibers can be especially prone to these defects.
What is needed, therefore, is a dielectric substrate having a thin non-woven fibrous mat material incorporated therein that has good mechanical and electrical properties, for example, low coefficient of thermal expansion (CTE) in all directions and low dissipation factor.